If you want a really hacky version, then if you didn't save Barcus Wroot, he should be dead by the windmill in Blighted Village by the time you get to Grymforge. You can push his body to the Underdark through the Spider Lair. Then raise him with Glut, and he'll act like you saved him. Then both he, and the character he is attached to, will teleport to Grymforge. You can then go and find the waypoint.
